Illumination device and display device

ABSTRACT

According to one embodiment, the illumination device includes a first area, a second area, a first light guide, a plurality of first light sources, a second light guide, a plurality of second light sources, a first layer and a second layer. The first layer extends from the first area over a boundary between the first area and the second area, to between a second side surface of the first light guide and the boundary. The second layer extends from the second area over the boundary to between a third side surface of the second light guide and the boundary.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2019-144558, filed Aug. 6, 2019, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to an illumination deviceand a display device.

BACKGROUND

Display devices such as liquid crystal display devices, for example,comprise a display panel containing pixels and an illumination devicesuch as a backlight for illuminating the display panel. The illuminationdevice comprises a light source which emits light and a light guide towhich light from the light source is irradiated. The light from thelight source enters the light guide from a side surface thereof,propagates inside the light guide, and is emitted from an emissionsurface which corresponds to one of main surfaces of the light guide.

When non-uniformity of luminance occurs in the emission surface of thelight guide, the quality of images displayed by the display panel can bedegraded as well. For example, when the viewing angle of light emittedfrom the light source is narrow, a desired luminance may not be obtainedin a region close to the light source on the emission surface of thelight guide. In this case, in the region near the light source of theemission surface, light of a luminance sufficient for the display panelcannot be supplied.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a decomposed perspective view showing a configuration exampleof a display device of an embodiment.

FIG. 2 is a plan view of an illumination device shown in FIG. 1.

FIG. 3 is a cross-sectional view showing a display device shown in FIG.1.

FIG. 4 is a diagram illustrating shapes of a first layer and a secondlayer, which is a perspective view of the illumination device shown inFIG. 3.

FIG. 5 is a partial cross section of the light guide, the first layerand the light source shown in FIG. 3.

FIG. 6 is a cross section of the illumination device near the boundary.

FIG. 7 is a cross section of the first layer and the light guide nearthe side surface.

FIG. 8 is a cross section of a modified example of the embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, there is provided anillumination device comprising: a first area; a second area adjacent tothe first area in a first direction; a first light guide comprising afirst main surface located in the first area and the second area, asecond main surface located in the first area and the second area, on anopposite side to the first main surface, a first side surface located inthe first area and a second side surface located in the second area onan opposite side to the first side surface in the first direction; aplurality of first light sources opposing the second side surface; asecond light guide comprising a third main surface opposing the secondmain surface and located in the first area and the second area, a fourthmain surface located in the first area and the second area, on anopposite side to the third main surface, a third side surface located inthe first area, and a fourth side surface located in the second area andopposing the third side surface in the first direction; a plurality ofsecond light sources opposing the third side surface; a first layerincluding a plurality of first prisms provided on the second mainsurface; and a second layer including a plurality of second prismsprovided on the fourth main surface. The first layer extends from thefirst area over a boundary between the first area and the second area tobetween the second side surface and the boundary, and the second layerextends from the second area over the boundary to between the third sidesurface and the boundary.

According to another embodiment, there is provided a display devicecomprising: an illumination device; and a display panel which displaysimages. The illumination device comprises a first area; a second areaadjacent to the first area in a first direction; a first light guidecomprising a first main surface located in the first area and the secondarea, a second main surface located in the first area and the secondarea, on an opposite side to the first main surface, a first sidesurface located in the first area and a second side surface located inthe second area on an opposite side to the first side surface in thefirst direction; a plurality of first light sources opposing the secondside surface; a second light guide comprising a third main surfaceopposing the second main surface and located in the first area and thesecond area, a fourth main surface located in the first area and thesecond area, on an opposite side to the third main surface, a third sidesurface located in the first area, and a fourth side surface located inthe second area and opposing the third side surface in the firstdirection; a plurality of second light sources opposing the third sidesurface; a first layer including a plurality of first prisms provided onthe second main surface; and a second layer including a plurality ofsecond prisms provided on the fourth main surface. The first layerextending from the first area over a boundary between the first area andthe second area to between the second side surface and the boundary, andthe second layer extending from the second area over the boundary tobetween the third side surface and the boundary. The display paneloppose the first main surface.

Embodiments will be described hereinafter with reference to theaccompanying drawings. The disclosure is a mere example, and arbitrarychange of gist which can be easily conceived by a person of ordinaryskill in the art naturally falls within the inventive scope.

To more clarify the explanations, the drawings may pictorially showwidth, thickness, shape and the like, of each portion as compared withan actual aspect, but they are mere examples and do not restrict theinterpretation of the invention. In the present specification anddrawings, elements like or similar to those in the already describeddrawings may be denoted by similar reference numbers and their detaileddescriptions may be arbitrarily omitted.

In the embodiments, a transmissive liquid crystal display device will beexplained as an example of the display device DSP. Note that the mainconfiguration disclosed in the embodiments is also applicable to liquidcrystal display devices comprising a reflective function which reflectsexternal light to utilize the reflecting light for displaying, inaddition to the transmissive function, electronic-paper display devicescomprising an electrophoretic element, display devices which applymicro-electromechanical systems (MEMS), display devices which applyelectrochromism, or the like.

FIG. 1 is a decomposed perspective view showing a configuration exampleof a display device DSP according to an embodiment.

As shown in FIG. 1, a direction X, a direction Y and a direction Z areorthogonal to each other, but they may intersect with an angle otherthan 90 degrees. The direction X and the direction Y correspond todirections parallel to the main surface of a substrate which constitutesthe display device DSP, and the direction Z corresponds to a thicknessdirection of the display device DSP.

The display device DSP comprises a display panel PNL, an illuminationdevice IL, an IC chip 1 and a wiring substrate 2.

The display panel PNL comprises a first substrate SUB1 and a secondsubstrate SUB2. The display panel PNL includes a display area DA onwhich images are displayed. The display panel PNL includes, for example,a plurality of pixels PX arrayed in a matrix, in the display area DA.

The IC chip 1 and the wiring substrate 2 may read signals from thedisplay panel PNL, but they mainly function as signal sources whichsupply signals to the display panel PNL. The signal sources are mountedon one piece of the display panel PNL, in which the first substrate SUB1is exposed from second substrate SUB2. Note that the IC chip 1 may bemounted on the wiring substrate 2. The wiring substrate 2 is, forexample, a flexible printed board which can be bent.

The illumination device IL illuminates the display panel PNL. Theillumination device IL comprises a light guide LG1, a light guide LG2, aplurality of light sources LS1 and a plurality of light sources LS2. Thelight guide LG2, the light guide LG1, the first substrate SUB1 and thesecond substrate SUB2 are arranged in this order along the direction Z.

The light guide LG1 is formed into a flat plate parallel to an X-Y planedefined by the direction X and the direction Y. The light guide LG1comprises a main surface 1A opposing the display panel PNL, a mainsurface 1B on an opposite side to the main surface 1A, a side surfaceSF1 and a side surface SF2 on an opposite side to the side surface SF1.The side surface SF1 and the side surface SF2 are X-Z planes defined by,for example, the direction X and the direction Z. The light guide LG1has a thickness Tl. The thickness T1 is a length taken from the mainsurface 1A to the main surface 1B in the direction Z.

The light sources LS1 are arranged to be spaced apart from each other inthe direction X. The light sources LS1 each oppose the side surface SF2.

The light guide LG2 is formed into a flat plate parallel to the X-Yplane. The light guide LG2 comprises a main surface 2A opposing the mainsurface 1B, a main surface 2B on an opposite side to the main surface2A, a side surface SF3 and a side surface SF4 on an opposite side to theside surface SF3. The side surface SF3 and the side surface SF4 are, forexample, X-Z planes. The light guide LG2 has a thickness T2. Thethickness T2 is a length taken from the main surface 2A to the mainsurface 2B in the direction Z. In the example illustrated, the thicknessTl and the thickness T2 are substantially the same as each other. Notethat the thickness T1 and the thickness T2 may be same as or differentfrom each other.

The light sources LS2 are arranged to be spaced apart from each other inthe direction X. The light sources LS2 each oppose the side surface SF3.

The light sources LS1 and the light sources LS2 each are a laser lightsource such as a semiconductor laser which emits polarized laser beams,for example. Note that the light sources LS1 and the light sources LS2are not limited to those emitting laser beams, but they may be, forexample, light emitting diodes.

The light sources LS1 and the light sources LS2 each may comprise aplurality of light emitting elements emitting light of different colors,respectively. For example, if the light sources LS1 and the lightsources LS2 each comprise three light emitting elements emitting red,green, blue light, respectively, light of mixture of these colors (forexample, white) can be obtained.

FIG. 2 is a plan view of the illumination device IL shown in FIG. 1.

As shown in FIG. 2, the illumination device IL comprises a first areaA1, a second area A2 and a boundary BO between the first area A1 and thesecond area A2.

The first area A1 and the second area A2 are adjacent to each other inthe direction Y. The first area A1 has a length 10 in the direction Y,and the second area A2 has a length 20 in the direction Y. The length 10and the length 20 are substantially equal to each other. Note that thelength 10 and the length 20 may not necessarily be equal to each other.In the example illustrated, the light guide LG1 and the light guide LG2are located respectively over the entire regions of the first area A1and second area A2. In other words, the main surface 1A, the mainsurface 1B, the main surface 2A and the main surface 2B shown in theFIG. 2 are located in the first area A1 and the second area A2,respectively. The side surface SF1 and the side surface SF3 are locatedin the first area A1 and the side surface SF2 and the side surface SF4are located in the second area A2. The side surface SF1 and the sidesurface SF2 oppose each other in the direction Y and the side surfaceSF3 and the side surface SF4 oppose each other in the direction Y. Theside surface SF1 and the side surface SF3 overlap each other and theside surface SF2 and the side surface SF4 overlap each other. Theboundary BO is equivalent to each of a middle point between the sidesurface SF1 and the side surface SF2 and a middle point between the sidesurface SF3 and the side surface SF4.

The light sources LS1 each emit light in an emission direction DL1towards the side surface SF2. The intensity of light emitted from thelight sources LS1 is at maximum in an optical axis AX1, and the emissiondirection DL1 is parallel to the optical axis AX1. The light sources LS2each emit light in an emission direction DL2 towards the side surfaceSF3. The intensity of light emitted from the light sources LS2 is atmaximum in an optical axis AX2, and the emission direction DL2 isparallel to the optical axis AX2.

FIG. 3 is a cross-section showing the display device DSP shown in FIG.1.

As shown in FIG. 3, the display panel PNL further comprises a liquidcrystal layer LC, a sealant SE, a polarizer PL1 and a polarizer PL2.

The liquid crystal layer LC and the sealant SE are located between thefirst substrate SUB1 and the second substrate SUB2. The sealant SEadheres the first substrate SUB1 and the second substrate SUB2 togetherand also seals the liquid crystal layer LC.

The polarizer PL1 is adhered onto a lower surface of the first substrateSUB1. The second polarizer PL2 is adhered on an upper surface of thesecond substrate SUB2. A polarizing axis of the polarizer PL1 and apolarizing axis of the polarizer PL2 are, for example, orthogonal toeach other.

The illumination device IL further comprises a first layer P1, a secondlayer P2, a diffusion sheet DS, a prism sheet PS and a reflective sheetRS.

The diffusion sheet DS is located between the display panel PNL and thelight guide LG1. The diffusion sheet DS diffuses light entering thediffusion sheet DS to equalize the luminance of the light. The prismsheet PS is located between the diffusion sheet DS and the light guideLG1. The prism sheet PS concentrates the light emitted from, forexample, the main surface 1A of the light guide LG1 in the direction Z.The reflective sheet RS opposes the main surface 2B of the light guideLG2. The reflective sheet RS reflects light leaking from within thelight guide LG2, for example, to re-enter the light guide LG2.

The first layer P1 and the second layer P2 each are a layer including aplurality of prisms, which will be described later.

The first layer P1 is located on the main surface 1B. The first layer P1extends from the first area A1 over the boundary BO to between theboundary BO and the side surface SF2. The first layer P1 comprises anend portion E10 and an end portion E11 on an opposite side to the endportion E10. The end portion E10 is located between the boundary BO andthe side surface SF2, so as to be close to the boundary BO. The endportion E11 is close to the side surface SF1.

The second layer P2 is located on the main surface 2B. The second layerP2 extends from the second area A2 over the boundary BO to between theboundary BO and the side surface SF3. The second layer P2 has an endportion E20 and an end portion E21 on an opposite side to the endportion E20. The end portion E20 is located between the boundary BO andthe side surface SF3, so as to be close to the boundary BO. The endportion E21 is close to the side surface SF4. The first layer P1 and thesecond layer P2 overlap each other the boundary BO and in the vicinityof the boundary BO in the direction Z.

The light source LS1 is located remote from the side surface SF2. Theemission direction DL1 of the light source LS1 is a direction crossing anormal direction of the side surface SF2. The light source LS2 islocated remote from the side surface SF3. The emission direction DL2 ofthe light source LS2 is a direction crossing a normal direction of theside surface SF3.

Next, light beams L1 emitted by the light sources LS1 and light beams L2emitted by the light sources LS2 will be described.

The light beams L1 emitted by the light sources LS1 are refracted by theside surface SF2, so as to enter the light guide LG1. Of the light beamsL1 entering the light guide LG1, light beams traveling towards the mainsurface 1A are reflected by an interface between the light guide LG1 andan air layer. On the other hand, of the light beams L1 entering thelight guide LG1, light beams traveling towards the main surface 1B arereflected by an interface between the light guide LG1 and the air layer.Thus, in a region of the second area A2, where the first layer P1 is notprovided, the light beams L1 travel in the light guide LG1 whilerepeatedly reflected. Of the light beams L1 traveling in the light guideLG1, a travel direction of light advancing towards the first layer P1from the light guide LG1 is changed by a prism of the first layer P1 andthus the condition for total reflection of the main surface 1A isunapplied, thus enabling the emission of the light from the main surface1A. The light emitted from the main surface 1A illuminates the displaypanel PNL via the prism sheet PS and the diffusion sheet DS. That is, inthe region of the second area A2, where the first layer P1 is notprovided(, or in the vicinity of the side surface SF2), the incidentlight beams L1 are inhibited from entering the display panel PNL fromthe side surface SF2.

Similarly, light beams L2 emitted by the light sources LS2 are refractedby the side surface SF3 and enter the light guide LG2. In a region ofthe first area A1, where the second layer P2 is not provided, theincident light beams L2 advance in the light guide LG2 while repeatedlyreflected by the main surface 2A and the main surface 2B. Of the lighttraveling in the light guide LG2, a travelling direction of the lightadvancing towards the second layer P2 from the light guide LG2 ischanged by a prism of the second layer P2, and thus the condition fortotal reflection of the main surface 2A is unapplied, thus enabling theemission of the light from the main surface 2A. The light emitted fromthe main surface 2A illuminates the display panel PNL via the lightguide LG1, the prism sheet PS and the diffusion sheet DS. That is, inthe region of the first area A1, where the second layer P2 is notprovided(, or in the vicinity of the side surface SF3), the incidentlight beams L2 are inhibited from entering from the side surface SF3 tothe display panel PNL.

The display panel PNL is illuminated by the emission light beams L1 fromthe light sources LS1 in the first area A1. The display panel PNL isilluminated by the emission light beams L2 from the light source LS2 inthe second area A2.

Generally, the emission light beams from the light sources arranged atintervals each travel inside the light guide while scattering, but inthe vicinities of the light sources, the emission light beams do notsufficiently mix each other. Therefore, in a display device whichutilizes such light as illumination light, stripe-like non-uniformity inluminance and chromatic shift due to the difference in intensity may bevisually recognized when planarly viewed. The difference in intensitybetween the illumination light beams is reduced further as the locationis distant further from the light sources.

According to the embodiment, in the region of the second area A2, wherethe first layer P1 is not provided, the incident light beams L1 from theside surface SF2 are contained in the light guide LG1 and inhibited fromentering the display panel PNL. In the second area A2, the light beamsL1 from the light sources LS1 hardly enter the display panel PNL, butthe light beams L2 from the light sources LS2 illuminate the displaypanel PNL. The first area A1 is distant from the side surface SF2 by adistance sufficient for the light beams L1 to mix with each other. Thus,degradation of the illumination quality due to non-uniformity inluminance and the chromatic shift of the illumination light can besuppressed in the first area A1.

Similarly, in the region of the first area A1, where the second layer isnot provided, the incident light beams L2 from the side surface SF3 arecontained in the light guide LG2, and inhibited from entering thedisplay panel PNL. In the first area A1, the light beams L2 from thelight sources LS2 hardly enter the display panel PNL, but the lightbeams L1 from the light sources LS1 illuminate the display panel PNL.The second area A2 is distant from the side surface SF3 by a sufficientdistance for the light beams L2 to mix with each other. Thus,degradation of the illumination quality due to the non-uniformity of theillumination light can be suppressed in the second area A2.

Further, the first layer P1 extends over the boundary BO to the secondarea A2, and the second layer P2 extends over the boundary BO to thefirst area A1. Therefore, it is possible to avoid the reduction of theluminance level of the emission light of the illumination device IL invicinity of the boundary BO. Note that when the end portion El0 of thefirst layer P1 and the end portion E20 of the second layer P2 are eachlocated on the boundary BO, the luminance level of the emission light ofthe illumination device IL reduces in vicinity of the boundary BO.

FIG. 4 is a diagram illustrating the shape of the first layer P1 and thesecond layer P2, and is a perspective view of the illumination device ILshown in FIG. 3. FIG. 4 shows only a part of light guide LG1, a part ofthe light guide LG2, a part of the first layer

P1 and a part of the second layer P2 in the illumination device IL.

As shown in FIG. 4, the first layer P1 includes a plurality of prismsPA. The first layer P1 is constructed by the prisms PA arrangedintermittently in the direction Y. The second layer P2 includes aplurality of prisms PB. The second layer P2 is constructed by the prismsPB arranged intermittently in the direction Y. The prisms PA areprovided in the main surface 1B. The prisms PB are provided in the mainsurface 2B.

The prisms PA project from the main surface 1B towards the main surface2A. The prisms PA each have a cross section parallel to the Y-Z plane,which is triangular, and extend in the direction X. In the exampleillustrated, the cross sections of the prisms PA are similar to eachother. The prisms PA each comprise a slope SL1, a slope SL2, a referenceplane BL1, an intersection TL1 and a height HA. The slope SL1 is locatedon a side surface SF1 side and the slope SL2 is located on a sidesurface SF2 side. The reference plane BL1 is located on the plane samethat of the main surface 1B. The intersection TL1 is a line where theslope SL1 and the slope SL2 intersect each other. The intersections TL1are arranged at equal intervals 30 in the direction Y. The intervals 30are, for example, 0.1 mm. In the example illustrated, an angle θ11 madebetween the slope SL1 and the reference plane BL1 and an angle θ12between the slope SL2 and the reference plane BL1 are equal to eachother. Note that the angle θ11 corresponds to one of the interior anglesof the cross section of the prism PA, and the angle θ12 corresponds toanother one of the interior angles of the cross section of the prism PA.The cross section of the prism PA is an isosceles triangle. The heightHA is the height of the prism PA in the normal direction of the mainsurface 1B. The height HA is equivalent to the length taken from thereference plane BL1 to the intersection TL1 along the direction Z.

The height HA of the prism PA decreases as the location approaches fromthe side surface SF1 to the side surface SF2 (in a direction indicatedby an arrow pointing the direction Y). For example, the height HA of theprism PA becomes higher as the location approaches from the end portionE10 of the first layer P1 to the end portion E11 shown in FIG. 3. As thelocation approaches from the end portion E10 to the end portion E11, theratio of the prism PA (the reference plane BL1) per unit area in the X-Yplane increases and the ratio of the main surface 1B per unit area inthe X-Y plane decreases. Further, when the light travelling in the lightguide LG1 advances to the prism PA of the first layer P1 and then isemitted from the light guide LG1, the quantity of light traveling in thelight guide LG1 decreases. Thus, the illumination device IL canirradiate the illumination light of a uniform brightness distribution tothe display panel PNL in the first area A1.

The prisms PB each project from the main surface 2B in the direction Z.The prisms PB each have a cross section parallel to the Y-Z plane, whichis triangular, and extend in the direction X. In the exampleillustrated, the cross sections of the prisms PB are similar to eachother. The prisms PB each comprise a slope SL3, a slope SL4, a referenceplane BL2, an intersection TL2 and a height HB. The slope SL3 is locatedon a side surface SF3 side, and the slope SL4 is located on a sidesurface SF4 side. The reference plane BL2 is located on the plane sameas that of the main surface 2B. The intersection TL2 is a line where theslope SL3 and the slope SL4 intersect each other. The intersections TL2are arranged at equal intervals 30 along the direction Y. In the exampleillustrated, an angle θ13 made by the slope SL3 and the reference planeBL2 and an angle θ14 made by the slope SL4 and the reference plane BL2are equal to each other. Note that the angle θ13 corresponds to one ofthe interior angles in the cross section of the prism PB, and the angleθ14 corresponds to another one of the interior angles in the crosssection of the prism PB. The cross section of the prism PB is anisosceles triangle. The height HB is the height of the prism PB in thenormal direction of the main surface 2B. The height HB is equivalent tothe length taken from the reference plane BL2 to the intersection TL2along the direction Z.

The height HB of the prism PB decreases as the location approaches fromthe side surface SF4 to the side surface SF3 (in an opposite directionindicated by the arrow pointing the direction Y). As the locationapproaches from the side surface SF4 to the side surface SF3, the ratioof the prism PB (the reference plane BL2) per unit area in the X-Y planedecreases and the ratio of the main surface 2B per unit area in the X-Yplane increases. Further, when the light travelling in the light guideLG2 advances to the prism PB of the second layer P2 and then is emittedfrom the light guide LG2, the quantity of light traveling in the lightguide LG2 decreases. Thus, the illumination device IL can irradiate theillumination light of a uniform brightness distribution to the displaypanel PNL in the second area A1.

FIG. 5 is a partial cross-sectional view of the light guide LG1, thefirst layer P1 and a light source LS1 shown in FIG. 3.

As shown in FIG. 5, the light source LS1 comprises a light emissionpoint LP1 and a light emission surface LF1. The light emission point LP1is a point from where a light beam Ll having an optical axis AX1parallel to an emission direction DL1 is emitted. The light beam L1emitted from the light emission point LP1 travels while spreading. Theemission surface LF1 corresponds to, for example, an outer surface ofthe light source LS1.

The emission direction DL1 is inclined to the direction Y and thedirection Z. The emission direction DL1 and the side surface SF2 are notorthogonal to each other. That is, the emission direction DL1 intersectsa normal direction of the side surface SF2. With this configuration, thelight beam L1 is refracted when entering the light guide LG1. Anincident angle θ1 of the light beam L1 to the light guide LG1 is smallerthan an angle made by the emission direction DL1 and the direction Y.Further, the incident angle θ1 is equal to the angle θ11.

In the example illustrated, the light beam L1 traveling through thelight guide LG1 is reflected by the slope SL1 of the prism PA. The lightbeam L1 reflected by the slope SL1 is inapplicable to the totalreflection condition of the main surface 1A and is refracted by aninterface between the main surface 1A and the air layer, to be emittedat an emission angle θ2 from the main surface 1A. The emission angle θ2is an angle made by the light emitted from the main surface 1A and thenormal line of the main surface 1A. The refractive index of the lightguide LG1 is represented by n.

A relationship between the angle θ11 of the prism PA and the refractiveindex n of the emission angle θ2, can be expressed by the followingformula.

$\begin{matrix}{{\theta 11} = {\frac{1}{3}\left( {{90} - {\sin^{- 1}\left( {\frac{1}{n}\sin \; {\theta 2}} \right)}} \right)}} & {{Formula}\mspace{14mu} 1}\end{matrix}$

A structure similar to that of FIG. 5 can be applied to the side surfaceSF3 and the light sources LS2 as well. That is, the emission directionDL2 of the light source LS2 and the side surface SF3 are not orthogonal,but intersects the emission direction DL2 and the normal direction ofthe side surface SF3. Note that the structure of the side surface SF3and the light sources LS2 may be different from the structure shown inFIG. 5.

FIG. 6 is a cross section of the illumination device IL in the vicinityof the boundary BO.

As shown in FIG. 6, the first layer P1 has a length D1 in the secondarea A2. The length D1 is equivalent to a length taken from the boundaryBO to the end portion E10 in the direction Y. The second layer P2 has alength D2 in the first area A1. The length D2 is equivalent to a lengthtaken from the boundary BO to the end portion E20 in the direction Y. Inthe example illustrated, the length D2 is equal to twice the length D1.Note that it suffices if the length D2 is greater than the length D1.

A relationship between the length D1, the thickness T1, the emissionangle θ2 and the refractive index n can be expressed by the followingformula.

$\begin{matrix}{{D\; 1} = {2 \times T\; 1 \times \tan \left\{ {\sin^{- 1}\left( {\frac{1}{n}\sin \; {\theta 2}} \right)} \right\}}} & {{Formula}\mspace{14mu} 2}\end{matrix}$

The length 40 is a length taken from the main surface 2B to the mainsurface 1A in the direction Z. The length 40 is substantially a total ofthe thickness T1 and the thickness T2 (or twice the thickness T1). Inthe example illustrated, the angle made by the light beam L1 reflectedby the end portion E10 of the first layer P1 and the main surface 1B isangle θ3. The angle made by the light beam L2 reflected by the endportion E20 of the second layer P2 and the main surface 2B is angle θ4.The angles θ3 and θ4 are acute angles, respectively, and are equal toeach other.

According to this embodiment, the first layer P1 has a length D1 in thesecond area A2, and the second layer P2 has a length D2 in the firstarea A1. The light travelling through the light guide LG1 is emittedfrom the main surface 1A in the boundary BO by the prisms PA located inthe end portion E10. The light travelling through the light guide LG2 isemitted from the main surface 1A in the boundary BO by the prisms PBlocated in the end portion E20. With this configuration, as compared tothe case where the first layer P1 does not extend to the second area A2and the second layer P2 do not extend to the first area A1, theillumination device IL can also emit illumination light of a uniformluminance in the vicinity of the boundary BO as well.

FIG. 7 is a cross section of the first layer P1 and the light guide LG1in the vicinity of the side surface SF1.

As shown in FIG. 7, the end portion E11 of the first layer P1 is spacedapart from the side surface SF1 by a distance of a length D3 in thedirection Y. A relationship between the length D3, the thickness T1, theemission angle θ2 and the refractive index n, can be expressed by thefollowing formula.

$\begin{matrix}{{D\; 3} = {T\; 1 \times \tan \left\{ {\sin^{- 1}\left( {\frac{1}{n}\sin \; {\theta 2}} \right)} \right\}}} & {{Formula}\mspace{14mu} 3}\end{matrix}$

In the example illustrated, the light beam L1 traveling to the endportion E11 is reflected by the first layer P1, and then advancestowards the main surface 1A without advancing to the side surface SF1.Thus, the light beam Ll traveling in the light guide LG1 and thenreflected by the first layer P1 can be inhibited from being reflected bythe side surface SF1 thereafter, which may cause increase in luminanceof the illumination light of the illumination device IL partially in thevicinity of the side surface SF1.

A structure similar to that of FIG. 7 can be applied to the end portionE21 of the second layer P2 as well. That is, the end portion E21 of thesecond layer is spaced apart from the side surface SF4 by a distance ofthe length D3 in the direction Y.

In the configuration example described above, the direction Y isequivalent to the first direction, the direction X is equivalent to thesecond direction, the light sources LS1 are equivalent to the firstlight sources, the light sources LS2 are equivalent to the second lightsources, the light guide LG1 is equivalent to the first light guide, thelight guide LG2 is equivalent to the second light guide, the mainsurface 1A is equivalent to the first main surface, the main surface 1Bis equivalent to the second main surface, the main surface 2A isequivalent to the third main surface, the main surface 2B is equivalentto the fourth main surface, the side surfaces SF1 to SF4 are equivalentto the first to fourth sides, respectively, the prisms PA are equivalentto the first prisms, the prisms PB are equivalent to the second prisms,slopes SL1 to SL4 are equivalent to the first to fourth slopes,respectively, the intersection TL1 is equivalent to the firstintersection, the intersection TL2 is equivalent to the secondintersection, the reference plane BL1 is equivalent to the firstreference plane, the reference plane BL2 is equivalent to the secondreference plane, the end portion E10 is equivalent to the first endportion, the end portion E20 is equivalent to the second end portion,the length D1 is equivalent to the first length, and the length D2 isequivalent to the second length.

FIG. 8 is a cross section showing a modified example of the embodiment.

As shown in FIG. 8, a prism PA of the modified example is different froma prism PA of the embodiment in that the angle θ11 and the angle θ12 arenot equal to each other. The angle θ12 is greater than the angle θ11. Inthe example illustrated, the angle θ12 is 90 degrees, and the Y-Z crosssection of the prism PA is a right-angled triangle. The area of theslope SL1 is greater than the area of the slope SL2.

The light beam L1 travelling in the light guide LG1 advances in thelight guide LG1 while spreading. The diffused light EL shown in the FIG.8 is equivalent to the light spreading from the light beam L1. In themodified example illustrated, the area of the slope SL1 is greater ascompared to the case of FIG. 5, in which the cross section of the prismPA is a right-angled triangle. With this configuration, chances arehigher that the diffused light EL are reflected by the slope SL1 of theprism PA, thereby making it possible to increase the luminance of theillumination light of the illumination device IL.

A structure similar to that of FIG. 8 can be applied to the prism PB aswell. That is, the angle θ14 is greater than the angle θ13, and the areaof the slope SL4 is greater than the area of the slope SL3.

As explained above, according to the embodiments, an illumination devicewhich can suppress the deterioration in illumination quality and such adisplay device can be provided.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

What is claimed is:
 1. An illumination device comprising: a first area;a second area adjacent to the first area in a first direction; a firstlight guide comprising a first main surface located in the first areaand the second area, a second main surface located in the first area andthe second area, on an opposite side to the first main surface, a firstside surface located in the first area and a second side surface locatedin the second area on an opposite side to the first side surface in thefirst direction; a plurality of first light sources opposing the secondside surface; a second light guide comprising a third main surfaceopposing the second main surface and located in the first area and thesecond area, a fourth main surface located in the first area and thesecond area, on an opposite side to the third main surface, a third sidesurface located in the first area, and a fourth side surface located inthe second area and opposing the third side surface in the firstdirection; a plurality of second light sources opposing the third sidesurface; a first layer including a plurality of first prisms provided onthe second main surface; and a second layer including a plurality ofsecond prisms provided on the fourth main surface, the first layerextending from the first area over a boundary between the first area andthe second area to between the second side surface and the boundary, andthe second layer extending from the second area over the boundary tobetween the third side surface and the boundary.
 2. The device of claim1, wherein when a length of the first layer in the second area in thefirst direction is defined as a first length and a length of the secondlayer in the first area in the first direction is defined as a secondlength, the second length is greater than the first length.
 3. Thedevice of claim 2, wherein the second length is twice the first length.4. The device of claim 1, wherein the plurality of first prisms eachhave a triangular cross section, extend in a second direction whichintersects the first direction, and comprise a first slope on a firstside surface side, a second slope on a second side surface side, and afirst intersection where the first slope and the second slope intersecteach other, the plurality of second prisms each have a triangular crosssection, extend in the second direction, and comprise a third slope on athird side surface side, a fourth slope on a fourth side surface side,and a second intersection where the third slope and the fourth slopeintersect each other, the first intersections are arranged along thefirst direction at equal intervals, the second intersections arearranged along the first direction at equal intervals, a height of thefirst prisms in a normal direction of the second main surface decreasesfrom the first side surface towards the second side surface, and aheight of the second prisms in a normal direction of the fourth mainsurface decreases from the fourth side surface towards the third sidesurface.
 5. The device of claim 4, wherein the plurality of first prismseach comprise a first reference plane located on a plane same as that ofthe second main surface, the plurality of second prisms each comprise asecond reference plane located on a plane same as that of the fourthmain surface, an angle made by the second slope and the first referenceplane is greater than or equal to an angle made by the first slope andthe first reference plane, and an angle made by the third slope and thesecond reference plane is greater than or equal to an angle made by thefourth slope and the second reference plane.
 6. The device of claim 1,wherein the first layer includes a first end portion located close tothe first side surface, the second layer includes a second end portionlocated close to the fourth side surface, the first end portion isspaced away from the first side surface in the first direction, and thesecond end portion is spaced away from the fourth side surface in thefirst direction.
 7. The device of claim 1, wherein the plurality offirst light sources and the plurality of second light sources each are alaser light source.
 8. A display device comprising: an illuminationdevice; and a display panel which displays images, the illuminationdevice comprising: a first area; a second area adjacent to the firstarea in a first direction; a first light guide comprising a first mainsurface located in the first area and the second area, a second mainsurface located in the first area and the second area, on an oppositeside to the first main surface, a first side surface located in thefirst area and a second side surface located in the second area on anopposite side to the first side surface in the first direction; aplurality of first light sources opposing the second side surface; asecond light guide comprising a third main surface opposing the secondmain surface and located in the first area and the second area, a fourthmain surface located in the first area and the second area, on anopposite side to the third main surface, a third side surface located inthe first area, and a fourth side surface located in the second area andopposing the third side surface in the first direction; a plurality ofsecond light sources opposing the third side surface; a first layerincluding a plurality of first prisms provided on the second mainsurface; and a second layer including a plurality of second prismsprovided on the fourth main surface, the first layer extending from thefirst area over a boundary between the first area and the second area tobetween the second side surface and the boundary, the second layerextending from the second area over the boundary to between the thirdside surface and the boundary, and the display panel opposing the firstmain surface.
 9. The device of claim 8, wherein when a length of thefirst layer in the second area in the first direction is defined as afirst length and a length of the second layer in the first area in thefirst direction is defined as a second length, the second length isgreater than the first length.
 10. The device of claim 9, wherein thesecond length is twice the first length.
 11. The device of claim 8,wherein the plurality of first prisms each have a triangular crosssection, extend in a second direction which intersects the firstdirection, and comprise a first slope on a first side surface side, asecond slope on a second side surface side, and a first intersectionwhere the first slope and the second slope intersect each other, theplurality of second prisms each have a triangular cross section, extendin the second direction, and comprise a third slope on a third sidesurface side, a fourth slope on a fourth side surface side, and a secondintersection where the third slope and the fourth slope intersect eachother, the first intersections are arranged along the first direction atequal intervals, the second intersections are arranged along the firstdirection at equal intervals, a height of the first prisms in a normaldirection of the second main surface decreases from the first sidesurface towards the second side surface, and a height of the secondprisms in a normal direction of the fourth main surface decreases fromthe fourth side surface towards the third side surface.
 12. The deviceof claim 11, wherein the plurality of first prisms each comprise a firstreference plane located on a plane same as that of the second mainsurface, the plurality of second prisms each comprise a second referenceplane located on a plane same as that of the fourth main surface, anangle made by the second slope and the first reference plane is greaterthan or equal to an angle made by the first slope and the firstreference plane, and an angle made by the third slope and the secondreference plane is greater than or equal to an angle made by the fourthslope and the second reference plane.
 13. The device of claim 8, whereinthe first layer includes a first end portion located close to the firstside surface, the second layer includes a second end portion locatedclose to the fourth side surface, the first end portion is spaced awayfrom the first side surface in the first direction, and the second endportion is spaced away from the fourth side surface in the firstdirection.
 14. The device of claim 8, wherein the plurality of firstlight sources and the plurality of second light sources each are a laserlight source.